yocto: Migrate to genericarm64 from OE-core

In scarthgap, genericarm64 is now in the meta-yocto-bsp layer in
OE-core, which replaces generic-arm64 in meta-arm.

Add meta-yocto-bsp to the virtualization stack layers and rename
generic-arm64 to genericarm64 in all occurrences.

In Yocto scarthgap's genericarm64 BSP, CONFIG_PCI_HOST_GENERIC defaults
to a kernel module instead of a builtin. This is required to demonstrate
the PCI passthrough feature so install its package in DomUs.

genericarm64 by default builds an initramfs bundle, so override
INITRAMFS_IMAGE to "".

Signed-off-by: Peter Hoyes <peter.hoyes@arm.com>